1. Field of the Invention
This invention relates to inflatable passive restraint systems used in vehicles for restraining the movement of a seated occupant during a collision and, more particularly, to improved igniters for the activation of the gas generator, or inflator, of such a system.
2. Description of Related Art
Safety restraint systems which self-actuate from an undeployed state to a deployed state without the need for intervention by the operator, i.e. "passive restraint systems", and particularly those restraint systems incorporating inflatable bags or cushions (commonly referred to as "airbags") have been devised for automotive vehicles. In such systems, one or more airbags are stowed in storage areas within the passenger compartment of the vehicle. Upon actuation, the airbag is deployed from its storage area into the passenger compartment through openings in the vehicle interior.
Vehicular inflatable restraint systems normally include at least one crash sensor, generally positioned about the frame and/or body of the subject vehicle, which serves to sense sudden decelerations by the vehicle. Upon detection of such a sudden deceleration, as would occur in a collision, the airbag or airbags are rapidly inflated and deployed to cushion the driver and/or passengers from injury-causing contact with the interior structure of the vehicle.
An airbag provided for the protection of the vehicle driver, i.e. a driver side airbag, is usually mounted in a storage compartment located in the steering column of the vehicle. An airbag for the protection of a front seat passenger, i.e., a passenger side airbag, is typically mounted in a storage compartment behind the instrument panel/dashboard of the vehicle. Such airbags are housed in a deflated, folded condition to minimize space requirements. Mounted in close conjunction with the airbags are the necessary elements to cause inflation thereof upon actuation by the sensor. A number of devices are known for the inflation of airbags. In one device, known as an inert gas inflator, a folded airbag is inflated by gas supplied from a container charged with a pressurized supply of inert gas. In another known device, referred to as a pyrotechnic inflator, gas is generated by ignition of a pyrogenic gas generating composition, the components of which either decompose or chemically react to generate sufficient gas to inflate one or more airbags in the required time period. A third type of device, known as a hybrid inflator, uses both a stored gas supply and a pyrogenic gas generating composition. In those devices using gas generating compositions, an igniter is required to initiate the generation of the inflation gas. Igniters may take a number of forms dependent on the specific gas generation device and the particular gas generant composition utilized.
One form of igniter is described in U.S. Pat. No. 4,005,876, of Jorgensen et al., issued Feb. 1, 1977. In the device described in this patent, the igniter includes a steel tube filled with a pyrotechnic material, an electric squib at one end of the tube, and a long fuse extending the length of the tube. The tube contains an igniter composition comprising a granular mixture of boron and potassium nitrate. Pellets of a gas generating composition, such as sodium azide, surround the steel tube. The electric squib initiates combustion of the fuse, igniting the igniter composition which releases a flame of hot gases on the gas generant pellets.
Hamilton, in U.S. Pat. No. 4,200,615 issued Apr. 29, 1980, teaches a similar linear igniter.
A further device is taught in Cunningham, U.S. Pat. No. 4,878,690, issued Nov. 7, 1989. The igniter in this device comprises a relatively thin perforated aluminum or stainless steel tube. Rapid deflagration cord (RDC) is provided in the center of the tube with granules of an ignition composition surrounding it. The tube is covered with an adhesive backed aluminum foil to keep the ignition powder or granules from coming through the holes in the perforated steel tube. Foam plugs are press fit into the ends of the tube. In inflators which use wafers of the gas generant, the ignition tube is located in the center hole of the wafers, while in inflators which use particles or pellets of gas generant material, the igniter is centered in the combustion chamber and surrounded by the pellets. In such devices, ignition begins at the center of the tube and the ignition products flow through the perforations in the tube to the pyrogenic gas generant, heating it to a temperature at which it generates inflation gas. The initially generated inflation gas passes through the particles, pellets or wafers of gas generant to exit the combustion chamber through perforations provided in its exterior wall.
These prior art inflators all provide for passing the gas through a filter as they leave the combustion chamber and before they are discharged in the airbag. The filter serves both as a heat sink, cooling the gases, and to remove particulates from the gas stream. Such particulates can result from degradation of the gas generant wafers or pellets caused, at least in part, by the flow of the initially generated gas through the as yet unconsumed gas generant particles as such gas passes to the perforations in the exterior wall of the combustion chamber. Further particulates can also result from the partial deterioration of structural components used within the combustion chamber, the igniter and the squib initiator. The filters used in the prior art inflators effectively assure that such particulates do not exit into the airbag.
The prior art igniters, whether used with gas generant pellets or with gas generant wafers, are assembled to be centered in the inflator combustion chamber. Moreover, to be effective, the igniters must provide for an even ignition throughout the length of the inflator. To accomplish such they often use a rapid deflagration cord (RDC) fuse, having a flame propagation rate in the range of 660 to 1200 feet/second, surrounded by a binary ignition material, a mixture of finely divided boron and potassium nitrate, which provides a suitably high temperature impingement of ignition gases on the gas generant for an effective dwell time. Many variations of the prior art inflators and their linear igniters are presently in use and work well. However, the igniters are comprised of many components, some of which are costly, and require labor intensive assembly.
The igniters of the prior art are relatively expensive to make and install. Additionally, they can produce a hot particulate residue which must be removed to prevent its damaging the airbag during the inflation thereof. A need exists for less complicated, more easily assembled igniters which provide uniform ignition, while providing a high degree of safety and consistency.
Further, in many prior art inflators, specifically those made of aluminum, the possibility that they may be subjected to a high temperature environment, such as might be encountered in a vehicle fire, leads to a further safety concern. The gas generant in the inflator typically will auto ignite (ignite in the absence of actuation by the igniter) at a temperature of about 650.degree. F. (343.degree. C.). At such a temperature the strength of the aluminum housing degrades resulting in the possibility of it rupturing and scattering fragments in all directions. Since the use of relatively light weight aluminum in the housing offers weight advantages, the provision of an igniter which will auto-ignite, at a temperature at which the physical strength and integrity of an aluminum housing is not compromised, is desirable. Auto-ignition devices have been provided in prior art aluminum inflators, see U.S. Pat. No. 4,561,675, to Adams et al. and assigned to the assignee of this application. However, the provision of such auto-ignition means has required the assembly of the several components involved therein and their mounting in the inflator. A need exists for a simplified auto-ignition device with fewer components which can be assembled easily and less expensively.